drm/i915: Beef up skl_sanitize_cdclk() a bit

Also verify the DPLL_CTRL1 register value in skl_sanitize_cdclk(), throw
out a few unneeded variables, and write the CDCLK_CTL check a bit more
legible way.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-10-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>